1. Field of the Invention
The present invention relates to a control of a belt-type continuously variable transmission. Especially, the present invention relates to a gear change oil pressure control system for controlling a primary pulley's oil pressure independently of a secondary pulley's oil pressure.
2. Description of the Related Art
U.S. Pat. No. 6,338,695 (equivalent of Japanese Patent Unexamined Publication No. 2001-32916 [=JP2001032916]) discloses a gear change oil pressure control system of an automatic transmission (referred to as “vehicular transmission control system in English abstract”). The gear change oil pressure control system has two control valves, including a control valve for controlling a primary pulley oil pressure and a control valve for controlling a secondary pulley oil pressure. In the above system, an oil pressure required for a primary pulley is controllable independently of an oil pressure required for a secondary pulley.
According to U.S. Pat. No. 6,338,695 (equivalent of Japanese Patent Unexamined Publication No. 2001-32916 [=JP2001032916]), however, the oil pressure of one of the pulleys that is in need for a high oil pressure at a gear change is expected to obtain a higher oil pressure than the other of the pulleys. Hereinafter described is an assumed state that a pulley ratio is changed from an over drive (OD) side to a lowest side with a kick down and the like.
FIG. 6(a) shows a time chart including variation in the primary pulley oil pressure and the secondary pulley oil pressure. The pulley ratio in the over drive state keeps the primary pulley oil pressure higher than the secondary pulley oil pressure. Then, with an accelerator stepped on and thereby a kick down gear change instruction inputted, the primary pulley oil pressure may be set higher than in the over drive state, such that a belt slippage can be prevented. Moreover, the secondary pulley oil pressure is set still higher than the primary pulley oil pressure which is set high for promoting the gear change. With this, a differential thrust force can be secured, thus achieving at last a lowest gear change. This may be in need for a pump capable of generating a high oil pressure, increasing a pump loss and the like.